The present invention relates, in general, to a method and apparatus for packaging and cooling Tape Automated Bonding (TAB) type integrated circuit chips.
Removal of heat from integrated circuit (IC) chips is a central problem in modern high performance electronic packaging, requiring careful engineering. Often the well known or standard methods of thermal management are at odds with other package engineering or performance requirements. Incorporation of heat dissipation structures in a circuit chip module often undesirably increases complexity and size of the mechanical structure. The alternative is to design a structure which is usually not very efficient at removing heat from the circuit chips, and thus increases the likelihood of temperature related structural and electrical failures.
A number of different techniques have been proposed to avoid these problems. Usually these techniques involve the use of the chip module housing or package as a heat sink. To insure maximum cooling of the chips contained within the housing, the chips are mechanically connected to the housing or heat sink in a manner so that there is good thermal conductivity between them. This can pose a problem, however, since a rigid mechanical connection between the housing and the circuit chips can cause excessive stress on the chips which could damage them. If, on the other hand, a less rigid mechanical connection is employed, the efficiency of heat transfer from the chips to the heat sink will be reduced, and the probability of chip malfunction due to excessive operating temperature will be correspondingly increased.
Another problem presented by the necessity of coupling the circuit chips to a common heat sink or housing is that the dimensional tolerances from chip to chip on the mounting substrate are such that it is difficult to insure that every chip in the module will be coupled to the housing evenly. Some chips may not even be coupled at all to the housing, while excessive mechanical stress may be imparted to other chips.
There have been proposed a number of solutions in the past to these problems. One such solution is used by IBM in their Thermal Conduction Module for packaging and cooling IC's, and employs captive pistons within the heat sink to contact the chip backside, and accommodate variances in the mechanical features and tolerances. This technique is mechanically complex and therefore costly. Other techniques employ the use of thermally conductive material, such as solder or a thixotropic thermal compound to fill the gap between the chips and the heat sink. These techniques are difficult to implement, and there is little or no compliancy or spring action in the chip-heat sink subsystems to insure close mechanical and thermal contact, and accommodation of dimensional tolerance variations.
The present invention seeks to overcome the disadvantages of the prior techniques.